Reducing crosstalk between communications systems

ABSTRACT

Modems using a telephone line for high speed communications between them are arranged to monitor crosstalk from other existing communications systems with which they may mutually interfere, and adjust the power spectral densities (PSDs) of their transmitted signals to reduce overlap between the PSDs of the different systems, thereby reducing near end crosstalk. Communications can thereby be optimized for whatever crosstalk conditions may exist. The modems can have a master-slave relationship for communicating buffered frames in a half-duplex manner using a collision avoidance protocol for computer network access. Analysis of monitored crosstalk PSD information can be performed by each modem, by the master modem, or by a separate computer on the network. A digital signal processor used in each modem for receiving signals can be configured to be used at other times for the monitoring of crosstalk.

[0001] This is a continuation of pending U.S. patent application Ser.No. 08/691,486 filed Aug. 2, 1996 in the name of John B. Terry, entitled“Reducing Crosstalk Between Communications Systems”, the entiredisclosure of which is hereby incorporated herein by reference.

[0002] This invention relates to reducing crosstalk betweencommunications systems. The invention is particularly, but notexclusively, applicable to reducing NEXT (near end crosstalk) betweentwisted pairs of wires in telephone cables used historically forproviding telephone service to subscribers and now being usedincreasingly to provide additional communications services, for examplefor data communications and computer network connections.

REFERENCE TO RELATED APPLICATION

[0003] Reference is directed to U.S. patent application Ser. No.08/640,705 filed May 1, 1996 in the names of J. B. Terry et al.,entitled “Information Network Access Apparatus And Methods ForCommunicating Information Packets Via Telephone Lines”, the entiredisclosure of which is hereby incorporated herein by reference. Thisapplication, referred to below as the related application, describes andclaims methods and apparatus which can be used in particular tofacilitate remote access via conventional twisted pair telephone linesto computer networks such as the global computer information networkwhich is generally known as the Internet and is referred to herein asthe Network. The present invention is not limited in any way to thearrangements of this related application, but can be applied in aparticularly convenient manner to such arrangements as is describedlater below.

BACKGROUND OF THE INVENTION

[0004] Twisted pair public telephone lines are increasingly being usedto carry relatively high-speed signals instead of, or in addition to,telephone signals. Examples of such signals are ADSL (asymmetric digitalsubscriber line), HDSL (High Density Subscriber Line, T1 (1.544 Mb/s),and ISDN signals. There is a growing demand for increasing use oftelephone lines for high speed remote access to computer networks, andthere have been various proposals to address this demand, includingusing DOV (data over voice) systems to communicate signals via telephonelines at frequencies above the voice-band.

[0005] The provision in the public telephone network of varied servicesusing such diverse communications systems imposes a requirement thatdifferent and similar systems not interfere with one another. Apredominant limiting effect in this respect is NEXT (near end crosstalk)between wire pairs within multiple-pair cable binder groups or betweenwire pairs within adjacent binder groups.

[0006] Allocations of wire pairs within telephone cables in accordancewith service requests have typically resulted in a random distributionof pair utilization with few precise records of actual configurations.In addition, due to the nature of pair twisting in cables, and wherecable branching and splicing occurs, a wire pair can be in closeproximity to different other pairs over different parts of its length.At a telephone C.O. (central office), pairs in close proximity may becarrying diverse types of service using various modulation schemes, withconsiderable differences in signal levels (and receiver sensitivities)especially for pairs of considerably different lengths.

[0007] Statistical data has been developed that can be used to estimatecrosstalk between services using different pairs of multi-pair telephonecables, for example in terms of BER (bit error rate) based on powerspectral density (PSD, for example measured in milliwatts per Hertzexpressed in decibels, or dBm/Hz) overlap between the services. However,this statistical data is of limited use in practice in the provision ofa new service using equipment connected to a specific wire pair, in viewof factors such as those discussed in the preceding paragraph.

[0008] It is therefore a significant concern of telephone companies thatthe signals and operation of existing systems may be adversely affected,especially as a result of NEXT, by the deployment of new equipment,particularly digital signal transmission equipment. This concern isincreased in accordance with the extent to which such equipment islikely to be deployed, and hence particularly applies to equipment thatmay be used in very large numbers for remote access to computernetworks. New equipment can be designed in a manner largely to avoidinterference with other systems in accordance with the statistical data,but this imposes undesirable constraints on signal spectra and signallevels, limiting its usefulness in an unacceptable manner to accommodatea relatively small proportion of situations for which such constraintsmay be necessary.

[0009] An object of this invention is to provide a method that canpermit new communications systems to be added to existing communicationspaths in a manner that is generally compatible with existing systemswhere these exist, and that can make optimum use of communicationscapacity.

SUMMARY OF THE INVENTION

[0010] This invention provides a method of determining a power spectraldensity (PSD) for supplying signals from a signal transmitter to acommunications path, comprising the steps of: determining a PSD on thecommunications path, due to other communications, in the absence ofsignals supplied from the signal transmitter to the communications path;supplying signals from the signal transmitter to the communicationspath; and adjusting at least one parameter of the signals supplied fromthe signal transmitter to the communications path in dependence upon thedetermined PSD to reduce overlap between the PSD of the signals suppliedfrom the signal transmitter to the communications path and thedetermined PSD.

[0011] Thus a new communications system, operating in accordance withthis method, determines PSD on the communications path, primarily due toNEXT from other existing communications systems using adjacentcommunications paths, and then adjusts its own PSD to reduce, anddesirably to minimize, overlap between the PSDs. On the basis thatcrosstalk between different communications paths is equal for oppositedirections, the method consequently reduces, and desirably minimizes,interference from the new communications system with any existingcommunications systems that may be affected by the new system. Thus eachnew communications system that is deployed, for example in a publictelephone network where the communications paths comprise twisted pairtelephone lines, can be operated in a manner that is adaptively adjustedto minimize interference with existing systems in its own particularcommunications path environment. The adaptive adjustment can beperformed only once on deployment of the new system, or much moredesirably in an ongoing manner.

[0012] The step of determining a PSD on the communications path cancomprise monitoring a PSD on the communications path, while signals arenot supplied from the signal transmitter to the communications path, toproduce the determined PSD. As an alternative to not supplying signalsfrom the signal transmitter to the communications path during themonitoring, the PSD of signals supplied from the signal transmitter tothe communications path could be subtracted from the monitored PSDrepresenting the PSD on the communications path, due to othercommunications, in the absence of signals supplied from the signaltransmitter to the communications path.

[0013] Thus the determined PSD can be constituted by the monitored PSD.Such a determination can be valid where the existing communicationssystems are symmetric systems, for which the PSD of signals havingopposite directions of transmission can be substantially the same, butcan be inaccurate for asymmetric systems for which the PSD of signalshaving opposite directions of transmission can be substantiallydifferent. For example, in an ADSL system the spectral utilization, andhence the PSDs, of signals in the two opposite directions oftransmission are substantially different.

[0014] In view of this, preferably the step of determining a PSD on thecommunications path comprises the steps of: storing PSD templates forcommunications systems; monitoring a PSD on the communications path, dueto other communications, in the absence of signals supplied from thesignal transmitter to the communications path; comparing the monitoredPSD on the communications path with the templates to identify acommunications system corresponding to the monitored PSD; and producingthe determined PSD in dependence upon the identified communicationssystem. This enables the PSD of signals supplied from the signaltransmitter to the communications path to be adjusted to reduce overlapwith the PSD of signals of an existing system transmitted in theopposite direction of an adjacent communications path, this beingappropriate because of the predominance of NEXT.

[0015] The at least one parameter that is adjusted to reduce PSD overlapcan comprise the power (i.e. level), frequency band, and/or modulationscheme of signals supplied from the signal transmitter to thecommunications path. Desirably, all of these parameters are adjustedcollectively to achieve minimal interference with existingcommunications systems consistent with optimal performance of the newcommunications system.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The invention will be further understood from the followingdescription with reference to the accompanying drawings, in which:

[0017]FIG. 1 illustrates a communications arrangement using twisted pairtelephone lines for communicating high-speed signals, to which theinvention is particularly applicable;

[0018]FIG. 2 illustrates a communications arrangement in accordance withan embodiment of this invention;

[0019]FIG. 3 illustrates parts of a modem used in the arrangement ofFIG. 2; and

[0020]FIG. 4 illustrates a flow chart with reference to which operationof an arrangement in accordance with an embodiment of the invention isexplained.

DETAILED DESCRIPTION

[0021] Referring to FIG. 1, a line 10 represents a telephone line whichit is desired to use to provide communications between a modem 12,connected to the line 10 at a telephone C.O. end thereof, and acomplementary modem 14 connected to the line 10 at a subscriber endthereof. As is well known, diplexing filters (not shown) can be providedat the ends of the line 10 to permit the line to carry telephonecommunications simultaneously with communications at higher frequenciesbetween the modems 12 and 14. For simplicity, associated telephoneequipment is not shown in FIG. 1.

[0022] The telephone line 10 physically comprises a twisted wire pairwhich is typically in close proximity along at least part of its lengthwith other such twisted pairs in one or more multi-pair cables. Theseother pairs may carry arbitrary communications signals, includingtelephone and high-speed data signals, which as a result of crosstalkmay interfere with, and may be interfered with by, signals at similarfrequencies on the line 10. As discussed in the background of theinvention, the environment of the telephone lines and associated recordsmake it impractical to determine what communications equipment may beconnected to which twisted pairs over what parts of their lengths, sothat the nature and extent of any such interference can not generally bepredetermined. Because the deployment of high-speed communicationssystems is relatively small compared to the total deployment oftelephone lines, there frequently may be no significant interference. Insome cases, however, there is potential for interference with anexisting communications service.

[0023] For example, FIG. 1 illustrates that over part of its length theline 10 may be in close proximity to another telephone line 16 which isused for high-speed communications between a modem 18 at the C.O. end ofthe line 16 and a modem 20 at a subscriber end of this line. In thedescription below it is assumed, for example and in order to describeprinciples of the invention fully, that the existing modems 18 and 20provide for ADSL communications via the line 16, but the invention isapplicable regardless of the nature of the communications signals on thelines. A line 22 represents other telephone lines which may or may notcarry potentially interfering signals which would also be susceptible tointerference.

[0024] Because of the close proximity of the lines 10 and 12, crosstalkundesirably can occur between these lines; i.e. signals on one line arecoupled to some extent to the other line, the extent depending uponnumerous factors such as the physical characteristics of the cableincluding these lines, and the levels and frequencies (i.e. the powerspectral density) of the signals. Crosstalk is a particular concern forhigh-speed transmission, i.e. where both lines carry signals atrelatively high frequencies using similar or overlapping frequencybands. For the majority of telephone lines represented by the line 22,the addition of the modems 12 and 14 for high-speed communications viathe line 10 is not a significant problem as far as crosstalk isconcerned, because the majority of telephone lines do not also carryhigh frequency signals for high-speed transmission. For a relativelysmall number of telephone lines such as the line 16, the addition of themodems 12 and 14 for high-speed communications via the line 10 presentsa potential problem as far as crosstalk is concerned, because crosstalkfrom the line 10 to the line 16 can impair the signal transmission onthe line 16, and vice versa.

[0025] The main concern in this respect is NEXT, representedschematically in FIG. 1 by double-headed arrows 24 and 26 at the C.O.and subscriber ends, respectively, of the adjacent parts of the lines 10and 16. This can be seen to be the case from the fact that a signal fromthe modem 12 coupled by crosstalk from the nearest end of the line 10 tothe line 16 and back to the modem 18 is attenuated by the lines 10 and16 to a relatively small extent, whereas a signal transmitted from themodem 14 coupled by crosstalk from the line 10 to the line 16 andreceived by the modem 18 (FEXT or far end crosstalk) is subject tosignificant attenuation by the length of the lines 10 and 16.Conversely, a signal from the modem 14 coupled by crosstalk from thenearest end of the line 10 to the line 16 and back to the modem 20 isattenuated by the lines 10 and 16 to a relatively small extent, whereasa signal transmitted from the modem 12 coupled by crosstalk from theline 10 to the line 16 and received by the modem 20 is subject tosignificant attenuation by the length of the lines 10 and 16.Reciprocally, signals on the line 16 are also coupled by crosstalk tothe line 10, and again NEXT is the most significant factor in view ofthe relative attenuation of signals by the lengths of the lines overwhich they are communicated.

[0026] For explaining principles in accordance with which embodiments ofthis invention operate, it is assumed for example that signals from theADSL modem 18 to the ADSL modem 20 (referred to as the downstreamdirection) occupy a broad frequency band in a range of about 100 kHz toabout 1 MHz, and that signals from the ADSL modem 20 to the ADSL modem18 (referred to as the upstream direction) occupy a narrower frequencyband in a range of about 50 to about 150 kHz. These figures are givenonly for illustrative purposes and for convenience in explaining theinvention.

[0027] A desirable result that the invention facilitates achieving is topermit the addition and operation of the new modems 12 and 14 in amanner that reduces crosstalk between the line 10 and other lines 16 and22 to the extent that this is necessary to avoid interfering undesirablywith any communications, such as the ADSL communications between themodems 18 and 20, that may (but frequently will not) exist on such otherlines, while permitting the new modems 12 and 14 to communicate via theline 10 in an optimum manner, e.g. with the highest allowable signallevels and the greatest allowable frequency bandwidths.

[0028] To this end, a sequence of steps described below is carried outinitially and/or in an ongoing manner (e.g. periodically, irregularly asdesired, or dependent upon parameters such as traffic conditions). Thesesteps include measurements or monitoring of the line 10 to which themodems 12 and 14 are connected. As described initially below, it isconvenient for the modems themselves not to transmit signals to the line10 during measurement or monitoring periods, but there can bealternatives to this, and to other aspects of the immediately followingdescription, that are discussed later below.

[0029] The new modems 12 and 14 first suspend transmission of signals tothe line 10 during a monitoring period that can be relatively brief,e.g. 50 ms, or more protracted as desired or necessary. During one ormore such monitoring periods, in each of the modems 12 and 14,separately or simultaneously, the modem receiver performs a spectralanalysis of any signals that may be received via the line 10. Thisspectral analysis is conveniently performed by controlling the bandwidthand centre frequency of a receive bandpass digital filter in the modemto receive signals within a narrow bandwidth that is progressivelychanged over a desired spectrum, for example 20 kHz to 1 MHz, whilemonitoring the power level of any signal that is received. It can beappreciated that any such signal will be predominantly a result of NEXTfrom signals on adjacent lines 16 and 22, and that the power spectrumthat is constituted by this monitoring will represent the extent of thecrosstalk and will be characteristic of the type of communicationssignals contributing to this crosstalk.

[0030] Thus each of the modems 12 and 14 can determine not only theextent of crosstalk between the line 10 and any other lines 16 and 22,but also, from a comparison of the monitored power spectrum with storedtemplates of power spectra for various systems, suitably modified totake into account the known NEXT frequency characteristics of thetwisted pair cable, the type of communications system predominantlycontributing to such crosstalk. Each of the modems 12 and 14 thenadjusts the power spectral density (PSD) for signals that it willtransmit to the line 10 to minimize overlap with the PSD of signals forthe opposite direction of transmission for any determined communicationssystem contributing to the monitored NEXT. On the basis that crosstalkbetween the line 10 and each other line is reciprocal, this alsominimizes NEXT from signals on the line 10 between the modems 12 and 14to any such determined communications system, whereby the differentsystems can both operate with minimal interference between them. Themodems 12 and 14 then resume (or start) transmission of signals to theline 10 in accordance with the adjusted PSD.

[0031] More specifically, in this example the modem 12 will receive andmonitor a power spectrum having components in the broad frequency rangeof 100 kHz to 1 MHz of the downstream ADSL signals on the line 16, andwill determine from this that it must adjust the PSD of its transmittedsignals to avoid NEXT with upstream signals of an ADSL system which arereceived by the existing modem 18. It therefore adjusts the spectrum ofits transmitted signals to avoid the 50 to 150 kHz range of upstreamADSL signals, for example controlling a digital transmit filter toprovide a pass band from 150 kHz to 1 MHz, and transmits its signals atan appropriate level within this frequency band, where even at arelatively high power level they do not interfere with the ADSLcommunications on the line 16. Conversely, the modem 14 will receive andmonitor a power spectrum having components in the narrower frequencyrange of 50 to 150 kHz of the upstream ADSL signals on the line 16, andwill determine from this that it must adjust the PSD of its transmittedsignals to avoid NEXT with downstream signals of an ADSL system whichare received by the existing modem 20. It therefore adjusts the spectrumof its transmitted signals to avoid the 100 kHz to 1 MHz range ofdownstream ADSL signals, for example controlling a digital transmitfilter to provide a pass band from 50 to 100 kHz, and transmits itssignals at an appropriate level within this frequency band, where theyalso do not interfere with the ADSL communications on the line 16.

[0032] Where the monitored power spectrum relates to a symmetricalcommunications system, for example ISDN signals, rather than anasymmetrical system such as ADSL as described by way of example above,the adjusted PSDs for signals to be transmitted by the modems 12 and 14can be the same rather than different as described above.

[0033] Although as described above transmission of signals by the modems12 and 14 is suspended during each monitoring period, this need notnecessarily be the case. For example, for the monitoring by the modem12, it is not essential for transmission of signals on the line 10 fromthe modem 14 to be suspended, because the PSD of such signals at themodem 12 can be known and compensated for in the monitoring by the modem12. However, this would require the modem 12 to have separate facilitiesfor receiving the signals from the modem 14 and for monitoring purposes,in contrast to using the same receive filter at different times forreceiving signals and for spectral analysis as described above, which ismuch more preferable. Also, the modem 12 can conceivably be arranged tomonitor for NEXT at the same time that it is itself supplying signals tothe line 10, these signals having a known PSD which can be subtracted inthe monitoring and spectral analysis process. However, this may berelatively difficult to achieve in practice, especially because themonitored NEXT has a much lower power than the signals supplied to theline 10 by the modem 12. Accordingly, it is more desirable in practiceto suspend the transmission of signals by the modem doing the monitoringof NEXT, and preferably by both modems, during the monitoring.

[0034] In addition, although as described above the modems 12 and 14operate relatively independently of one another and each performs thenecessary spectral analysis, this need not be the case. Moreparticularly, and for example as described in greater detail below, themodem 14 may be subordinate to the modem 12 in a master-slaverelationship, the modem 14 performing monitoring and adjusting the PSDof its transmitted signals in response to command messages from themodem 12, the spectral analysis process being performed by the modem 12.As a further alternative, as described further below, the spectralanalysis for a plurality of lines 10 and associated C.O. modems 12(which may be multiplexed for different lines 10) and customer modems 14may be performed centrally by a separate computer unit whichcommunicates messages with the modems in a time multiplexed manner.

[0035] These alternatives are particularly advantageous in a networkaccess arrangement in accordance with the related application referredto above. In such an arrangement, access to a CSMA/CD (Carrier SenseMultiple Access with Collision Detection) network, such as the Networkusing Ethernet frames, is provided via a telephone line by providing amaster modem at the head end and a slave modem at the subscriber end ofthe line. The master modem provides a CSMA/CD interface to the Networkand controls half duplex communications with the slave modem via theline to avoid collisions of Ethernet frames on the line. The Ethernetframes are enveloped in frames on the line with error checkinginformation; control information between the modems is contained in thesame and/or in separate frames. The modulation method and signalbandwidth can be varied depending on errors to provide optimumcommunications capacity via any particular line, and a ratio of upstreamto downstream frames can be varied depending on buffer fills at themodems. The master modem can be multiplexed for multiple lines. Themodulated signal frequencies are above telephone signal frequencies sothat each line can be frequency diplexed for simultaneous telephonecommunications.

[0036]FIG. 2 illustrates such an arrangement for one subscriber. In FIG.2, the modem 12 at the C.O. end of the line 10 in FIG. 1 is constitutedby a master modem 32, and the modem 14 at the customer end of the line10 in FIG. 1 is constituted by a slave modem 34. The master and slavemodems 32 and 34 are coupled to the telephone line 10 via diplexingfilters (DF) 30, which serve in known manner to separate low frequencytelephone signals, communicated between the telephone C.O. (not shown)and a customer telephone (not shown) at the respective ends of the line10, from higher frequency signals between the modems 32 and 34, thesesignals being frequency-multiplexed on the line 10.

[0037] Each of the modems 32 and 34 includes an Ethernet interface ofknown form. At the customer end of the line 10, Ethernet (ENET) framescommunicated via the slave modem 34 are coupled to an Ethernet interface(E I/F) of known form in a terminal device (TD) 36 which may for examplebe constituted by a personal computer. Thus Ethernet frames arecommunicated between the slave modem 34 and the terminal device 36 inknown manner, for example using twisted pair wiring and the 10BASE-TCSMA/CD standard; this communication can be expanded in known mannerinto a more extensive LAN (local area network). At the C.O. end of theline, Ethernet frames communicated via the master modem 32 are coupledvia an Ethernet hub or switch 38 and a router 40 to the Network. TheEthernet hub or switch 38 and router 40 are both of known form. Inaddition, a spectral compatibility manager (SCM) 42, for exampleconstituted by a computer, is also connected to the Ethernet switch 38as shown or elsewhere in the Network. The function of the SCM 42 isdescribed later below.

[0038] As shown at the top of FIG. 2, Ethernet frames are thuscommunicated on the customer side of the slave modem 34 and on theNetwork side of the master modem 32. Between the modems 32 and 34,Ethernet frames are communicated using a point-to-point protocol whichuses collision avoidance and for convenience is referred to as ECAP(Ethernet Collision Avoidance Protocol). Reference is directed to therelated application for a detailed description of this, but it isoutlined below.

[0039] The master and slave modems buffer Ethernet frames to becommunicated downstream (from the master modem 32 to the slave modem 34)and upstream (from the slave modem 34 to the master modem 32). The ECAPcommunications of the buffered Ethernet frames involves half-duplextransmission in which the master modem 32 has priority and control overthe slave modem 34. Thus the master modem 32 determines when to sendinformation downstream via the line 10, and informs the slave modem 34when it is permitted to send information upstream via the line 10. Tofacilitate these communications, the information sent via the line 10comprises not only the data packets of Ethernet frames but also controlpackets downstream and response packets upstream between the master andslave modems. The data and control packets are incorporated into ECAPframes which can take various forms. Control units in the master andslave modems perform the necessary conversions between the Ethernetframes and ECAP data frames, and generate and respond to the ECAPcontrol and response frames. Each of the master and slave modems 32 and34 includes an Ethernet interface as described above and hence has aunique network address provided by this interface; these addresses areused to address messages (control and response packets) between themodems and can also be used for addressing the modems from other devicessuch as the SCM 42 as described below.

[0040] Each of the modems 32 and 34 includes a modulator, demodulator,and related functions that are conveniently implemented in known mannerusing one or more DSPs (digital signal processors) with analog-digitalconversion in known manner. The DSPs are conveniently controlled toprovide arbitrary different signal bandwidths, low frequency limits (or,equivalently, filter centre frequencies), modulation methods (forexample the DSPs are programmed to select any of a plurality ofmodulation methods such as QAM (quadrature amplitude modulation), QPSK(quadrature phase shift keying), and BPSK (binary phase shift keying)),and (e.g. for QAM) different numbers of bits per symbol. The programmingand control of DSPs in this manner is known in the art and need not befurther described here. However, it is observed that this programmingand control, and control of the signal levels transmitted from themodems to the line 10, provides extensive control over the powerspectral density (PSD) of signals supplied to the line 10.

[0041] It can be appreciated from the above outline that the collisionavoidance protocol ensures that the modems 32 and 34 operate in ahalf-duplex manner for communications between them via the line 10, withthe total transmission capacity of the line being shared between thedownstream and upstream directions of transmission. The protocolprovides for control of the signal bandwidth, modulation method, etc. toprovide a maximum throughput of Ethernet frames via the line 10 asdescribed in the related application. However, the same controlprinciples can be used in accordance with the present invention toadjust the PSD of signals supplied by the modems 32 and 34 to the line10 to reduce NEXT as described above with reference to FIG. 1.

[0042] Furthermore, it can be appreciated that the half-duplexcommunications between the modems 32 and 34 also provides, or can veryeasily provide, periods during which signals are not supplied to theline 10 and accordingly that can be used for monitoring the line 10 asdescribed above. For example, the control packets communicated from themaster modem 32 to the slave modem conveniently provide a facility forthe master modem 32 to instruct the slave modem 34 not to supply signalsto the line 10 for a given period, and to monitor the line 10 asdescribed above. During the same period, the master modem 32 similarlycan suspend supply of any signals to the line 10 and can monitor theline 10, whereby each modem monitors signals on the line 10 primarilydue to NEXT. Monitoring data from the slave modem 34 is thencommunicated in response packets to the master modem 32, so that onlythe master modem 32 performs a spectral analysis and the slave modem 34can be simplified accordingly (it must still be capable of monitoringNEXT PSD, but does not need to analyse the resulting data). As describedin the related application and indicated above, the master modem 32 isadvantageously used in a multiplexed manner for a plurality of lines 10and associated slave modems, and accordingly a single master modem canperform the spectral analysis, in an ongoing manner, for all of thelines 10 which it serves. In each case the master modem 32 then sets itsown PSD parameters, and via control packets commands the respectiveslave modem 34 to set its PSD parameters, in accordance with thedetermined PSD on the respective line 10 to minimize PSD overlap, andhence NEXT, as described above and to achieve an optimal (for theprevailing conditions applicable to that particular line 10) throughputof data frames as discussed above.

[0043] It can be appreciated that the ECAP communications establishedbetween the master and slave modems 32 and 34 provide a very simple andconvenient facility for both establishing silent periods for monitoringNEXT on the line 10 and adjusting the PSD of signals supplied by themodems to the line 10, not least because the Ethernet frames to becommunicated are already buffered in buffers in the modems. Thisprovides a distinct advantage over other arrangements using conventionalmodem communications, for which the establishment of silent periods formonitoring, and the control of the PSD of signals supplied to thetelephone line, may be considerably more complex.

[0044] As described above, the analysis of data provided by themonitoring of NEXT by both modems 12 and 14 can be carried out by themodem 32, and this can be multiplexed for a plurality of lines 10. Thismultiplexing or concentration of the analysis of data to determineappropriate PSD parameters for the modems can be further extended to becarried out by the SCM 42 instead of by the modems, with messages beingcommunicated between the SCM 42 and the modems accordingly. In thisrespect, the modems 32 and 34 can operate independently and can beaddressed individually, using their respective Ethernet addresses, forcommunications between the SCM 42 and the respective modem.Alternatively, as described below in greater detail, the SCM 42 cancommunicate with the master modem 32 at the C.O. end of the line 10, themaster modem 32 communicating with the slave modem 34 using ECAPcommunications as described above.

[0045] This is described further below with reference to FIG. 3,illustrating a block diagram form of the modems 32 and 34, and FIG. 4showing a flow chart.

[0046] Referring to FIG. 3, each of the modems 32 and 34 comprises ahybrid unit 50 connected (optionally via a diplexing filter 30, notshown in FIG. 3) to the telephone line 10 and an Ethernet interface(ENET I/F) 52 for connection to the terminal device 36 or Ethernetswitch 38. Analog signals received via the line 10 are supplied via thehybrid unit 50 to an analog-digital (A-D) converter 54 to be convertedinto digital form, the digital signals being passed via a configurabledigital signal processor (DSP) 56 to a buffer 58, which exchangescontrol (or response) information with a control unit 60 and data to bepassed on in Ethernet frames with the interface 52. In the oppositedirection, a buffer 62 exchanges control or response information withthe control unit 60 and Ethernet frame data with the interface 52, andinformation from the buffer 62 is supplied via a configurabletransmitter (Tx.) 64 and a digital-analog (D-A) converter 66 to thehybrid unit 50 and thence to the line 10. Digital components of themaster modem 32 can be multiplexed for a plurality of lines 10.

[0047] The control unit 60 controls the operation of the modem as eithera master modem 32 or a slave modem 34. For a master modem 32, Ethernetframes are exchanged with the Network from the buffers 58 and 62 via theinterface 52. The control unit 60 controls encapsulation into ECAPframes of Ethernet data frames from the buffer 62 and controlinformation which it generates for the slave modem 34, and controls thedownstream transmission of these via the transmitter 64, converter 66,hybrid unit 50, and the line 10. The control information includes pollswhich permit the slave modem 34 to transmit in the upstream direction,whereby the master modem ensures half duplex transmission on the line 10without collisions between the downstream and upstream transmissiondirections. Upstream ECAP frames are received via the hybrid unit 50,converter 54, and DSP 56, with response information being supplied tothe control unit 60 and Ethernet data frames being supplied via thebuffer 58 to the Ethernet interface 52.

[0048] Conversely, for a slave modem 34 ECAP frames on the line 10 arereceived via the hybrid unit 50, converter 54, and DSP 56, with controlinformation supplied to the slave modem's control unit 60 and Ethernetdata frames being supplied via the buffer 58 and Ethernet interface 52to the terminal device 36. In response to a poll in the controlinformation received from the master modem, the control unit 60 in theslave modem controls transmission upstream of one or more framescontaining response information and/or Ethernet data frames from thebuffer 62 in the slave modem, as instructed by the master modem 32.

[0049] The control unit 60 in each modem also controls the configurationof the DSP 56 and transmitter 64 of the modem. In particular, forexample, it controls parameters of the transmitter 64 such as the on/offstate, signal level, amplitude slope (variation in signal amplitude withfrequency over the pass band), centre frequency, and modulation scheme(e.g. QPSK or QAM and number of bits per symbol), which affect not onlythe transmission rate but also the PSD of the transmitted signal. Itcontrols similar parameters for the DSP 56 in a receive mode of themodem used for normal operation, and in a monitoring mode used formonitoring NEXT as described above it controls the DSP centre frequencyand bandwidth to provide for level measurement of any receivedcrosstalk.

[0050] The flow chart in FIG. 4 illustrates steps associated with thismonitoring, this in this case being controlled by the SCM 42 asindicated above. Each step is identified by a reference number that isgiven in parentheses in the following description. Steps 70 to 77 areperformed by the SCM 42 and are shown at the left of FIG. 4, steps 78 to92 are performed by the master modem 32 and are shown in the middle ofFIG. 4, and steps 93 to 98 are performed by the slave modem 34 and areshown at the right of FIG. 4.

[0051] Referring to FIG. 4, the SCM 42 initially selects (70) atelephone line 10 and direction to test, i.e. whether to monitor NEXT atthe master modem 32 or the slave modem 34 on the selected line, and thenselects (71) a centre frequency and bandwidth for this monitoring,sending (72) via the Network a message containing this information in anEthernet frame addressed to the master modem 32 using its address(determined by the Ethernet interface 52 of this master modem). Themaster modem 32 receives (78) this Ethernet frame and its control unit60 determines (79) whether the monitoring is to be carried out by theslave modem 34. If not, then the control unit 60 of the master modem 32suspends (80) transmission of frames downstream (thereby also suspendingpolling for the slave modem 34 so that frames are also not transmittedupstream on the line 10), configures the DSP 56 in accordance with theprovided message from the SCM 42 to perform (81) the desired measurementor monitoring of NEXT on the line 10, sends a resulting message in aconventional Ethernet frame addressed to the SCM 42 via the Network, andresumes (83) its transmission of frames downstream (and polling of theslave modem to permit upstream transmission).

[0052] The SCM 42 receives (73) the Ethernet frame containing themonitoring information and determines (74) whether a desired test hasbeen completed. If not, it returns to the step 71 and the above sequenceis repeated for another selected centre frequency and/or bandwidth. Ifthe test is complete, then the SCM 42 analyses (75) the monitoring dataprovided and determines (75) PSD parameters for signals sent to therespective line 10 by the respective modem to minimize interference withany other communications signals that it determines, in the mannerdescribed above, may be affected by crosstalk with signals from thisrespective modem. It then sends (76) an Ethernet frame addressed to themaster modem 32 containing a message with the determined parameters. Themaster modem 32 receives (84) this Ethernet frame and its control unit60 determines (85) whether the message is for the slave modem 34. Ifnot, then the control unit 60 of the master modem 32 adjusts (86) theconfiguration of its transmitter 64 in accordance with the PSDparameters provided, and sends (87) an Ethernet frame to the SCM 42 witha message confirming this adjustment. This is received (77) by the SCM42, which returns to the step 70. Obviously, these steps of the SCM 42can be carried out contemporaneously for many lines 10.

[0053] In the event that the master modem determines (79) that amonitoring message from the SCM 42 is intended for the slave modem, thenit sends (88) the message in an ECAP frame to the slave modem and thensuspends (80) its transmission of frames downstream. The slave modemreceives (93) this message, performs (94) the desired monitoring(without supplying signals to the line because it is not being polled todo so), and sends (95) the resulting monitoring information in an ECAPresponse frame to the master modem 32. The master modem receives (90)this information and sends (82) it to the SCM 42, continuing asdescribed above.

[0054] Similarly, in the event that the master modem determines (85)that a PSD adjustment message from the SCM 42 is intended for the slavemodem, then it sends (91) the message in an ECAP frame to the slavemodem, which receives (96) this message, adjusts (97) the configurationof its transmitter 64 in accordance with the PSD parameters provided,and sends (98) a message confirming this adjustment in an ECAP frame tothe master modem 32. This is received (92) by the master modem 32 andforwarded (87) to the SCM 42, continuing as described above.

[0055] It can be seen that in the manner described above analysis ofmonitoring data is performed centrally by the SCM 42 and can beperformed efficiently for many lines 10. In each case monitoring isperformed while signals are not supplied to the relevant line 10, theexisting DSP being configured for this purpose. The relatively briefmonitoring periods do not significantly interrupt the transmission ofinformation in either direction on the line 10, because this informationis already buffered in the buffer 62 in each modem. During themonitoring, Ethernet frames from the buffer 58 in each modem can stillbe supplied via the respective Ethernet interface 52. Furthermore, itcan be appreciated that brief monitoring periods may be establishedduring otherwise unused or idle periods of the half duplexcommunications on the line 10, without any extra interruption of theinformation transmission on the line 10, and/or that the same monitoringperiods can be used for monitoring at both ends of the line 10.

[0056] Within the constraints imposed by the PSD parameters provided bythe SCM 42 to reduce crosstalk, the master modem 34 can still optimizecommunications on the line 10 in the manner fully described in therelated application, for example controlling a ratio of upstream anddownstream frame transmission in dependence upon buffer fills.

[0057] Although as described above the slave modem 34 communicates withthe SCM 42 via the master modem 32, communications could instead becarried out using Ethernet frames addressed directly between the SCM 42and the slave modem 34. Such frames would, of course, still becommunicated via the master modem 32.

[0058] The invention has been described above in terms of a newcommunications system being provided, and adjusting the PSDs of itssignals, to be compatible with any existing system with which theremight otherwise be excessive interference. It can be appreciated thatthe same advantages can apply in respect of two or more new systems eachof which can adjust its PSDs so that they do not interfere with oneanother or with any other existing systems, so that multiple systems canco-exist in a compatible manner.

[0059] Thus although particular embodiments of the invention have beendescribed in detail, it should be appreciated that these and numerousother modifications, variations, and adaptations may be made withoutdeparting from the scope of the invention as defined in the claims.

What is claimed is:
 1. A method of determining a power spectral density(PSD) for supplying signals from a signal transmitter to acommunications path, comprising the steps of: determining a PSD on thecommunications path, due to other communications, in the absence ofsignals supplied from the signal transmitter to the communications path;supplying signals from the signal transmitter to the communicationspath; and adjusting at least one parameter of the signals supplied fromthe signal transmitter to the communications path in dependence upon thedetermined PSD to reduce overlap between the PSD of the signals suppliedfrom the signal transmitter to the communications path and thedetermined PSD.
 2. A method as claimed in claim 1 wherein the step ofdetermining a PSD on the communications path comprises monitoring a PSDon the communications path, while signals are not supplied from thesignal transmitter to the communications path, to produce the determinedPSD.
 3. A method as claimed in claim 1 wherein the step of determining aPSD on the communications path comprises the steps of: storing PSDtemplates for communications systems; monitoring a PSD on thecommunications path, due to other communications, in the absence ofsignals supplied from the signal transmitter to the communications path;comparing the monitored PSD on the communications path with thetemplates to identify a communications system corresponding to themonitored PSD; and producing the determined PSD in dependence upon theidentified communications system.
 4. A method as claimed in claim 3wherein the step of monitoring a PSD on the communications path isperformed signals are not supplied from the signal transmitter to thecommunications path.
 5. A method as claimed in claim 1 wherein said atleast one parameter comprises a power of the signals supplied from thesignal transmitter to the communications path.
 6. A method as claimed inclaim 1 wherein said at least one parameter comprises a frequency bandof the signals supplied from the signal transmitter to thecommunications path.
 7. A method as claimed in claim 1 wherein said atleast one parameter comprises a modulation scheme of the signalssupplied from the signal transmitter to the communications path.
 8. Amethod as claimed in claim 1 wherein said at least one parametercomprises a power, a frequency band, and a modulation scheme of thesignals supplied from the signal transmitter to the communications path.9. A method as claimed in claim 2 wherein said at least one parametercomprises a power of the signals supplied from the signal transmitter tothe communications path.
 10. A method as claimed in claim 2 wherein saidat least one parameter comprises a frequency band of the signalssupplied from the signal transmitter to the communications path.
 11. Amethod as claimed in claim 2 wherein said at least one parametercomprises a modulation scheme of the signals supplied from the signaltransmitter to the communications path.
 12. A method as claimed in claim2 wherein said at least one parameter comprises a power, a frequencyband, and a modulation scheme of the signals supplied from the signaltransmitter to the communications path.
 13. A method as claimed in claim3 wherein said at least one parameter comprises a power of the signalssupplied from the signal transmitter to the communications path.
 14. Amethod as claimed in claim 3 wherein said at least one parametercomprises a frequency band of the signals supplied from the signaltransmitter to the communications path.
 15. A method as claimed in claim3 wherein said at least one parameter comprises a modulation scheme ofthe signals supplied from the signal transmitter to the communicationspath.
 16. A method as claimed in claim 3 wherein said at least oneparameter comprises a power, a frequency band, and a modulation schemeof the signals supplied from the signal transmitter to thecommunications path.
 17. A method as claimed in claim 4 wherein said atleast one parameter comprises a power of the signals supplied from thesignal transmitter to the communications path.
 18. A method as claimedin claim 4 wherein said at least one parameter comprises a frequencyband of the signals supplied from the signal transmitter to thecommunications path.
 19. A method as claimed in claim 4 wherein said atleast one parameter comprises a modulation scheme of the signalssupplied from the signal transmitter to the communications path.
 20. Amethod as claimed in claim 4 wherein said at least one parametercomprises a power, a frequency band, and a modulation scheme of thesignals supplied from the signal transmitter to the communications path.